Hydraulic transmissions and particularly hydrostatic transmissions are in widespread use for a number of purposes and generally comprise pump means, consisting of one or more pumps, driven by a prime mover such as an internal-combustion engine or an electric motor, and motor means such as one or more motors hydraulically connected with the pump means and displacing an output shaft or a number of such shafts.
The transmission ratio of such transmissions can be established by providing the pump means with a control element which varies the displacement of the fluid per revolution. When the pump means is an axial-piston pump, this control element can be a tiltable plate or control disk which varies the stroke of the axial pistons of the cylinder barrel (see pages 113 ff. of FLUID POWER, U.S. Government Printing Office, Washington, D.C. 1966).
Frequently the hydraulic motors are also of the axial-piston type (op. cit. pages 199 ff.) and can have a variable out-put depending upon the position of the tiltable control element (usually the cylinder barrel) as well. In this case the variable displacement can be considered to be in terms of the fluid displacement per output revolution of the input shaft.
Such transmissions can be set such that the output shaft rotates at low speeds with very high torque, at high speeds with low torque or at any intermediate condition.
Transmissions of this type have received widespread application in industrial vehicles adapted to transport or control large loads. Typical of these applications is a fork-lift truck whose driving wheels may each be provided with a hydraulic motor forming part of a hydrostatic transmission of the type described. In these cases the hydraulic transmission has the advantage that it allows the vehicle to creep while carrying a relatively large load at a high position for precise stacking and to travel at high speeds when unloaded so as to move from place to place at a storage or other industrial facility.
In all cases the prior-art control arrangements had a selectively positionable control element for the hydrostatic transmission which would set the transmission ratio, i.e. the speed ratio between the input shaft of the pump means and the output shaft of the motor means as required by the vehicle operator, this element being coupled, for example, to a pedal or lever at the driver position.
In most instances this element positively and directly established a transmission ratio so that for a given input shaft speed the output shaft speed is always proportional to the setting of this element.
This has been found to be advantageous for many purposes and was believed heretofore to be the most advantageous arrangement for fork-lift trucks and the like. For example, with a fork-lift truck, when a heavy lead is lifted into a high position and must be set in place carefully, this arrangement permitted the vehicle speed to be meticulously controlled, even with nonlevel floors or traveling surfaces, so that the vehicle could creep into a position independently of resistance to displacement of the vehicle. In general, therefore, it is of considerable advantage to have direct control of the transmission ratio by the operator for hydrostatic transmissions at low output speeds, especially for fork-lift trucks. Of course, the transmission ratio can be altered sharply when higher output speeds are required and this was accomplished merely by resetting the operator-controlled element.
In another conventional transmission arrangement, the transmission ratio was made fully dependent upon the load at the output shaft, i.e. the required output torque. This had the advantage that with increasing loading of the output shaft, the output speed was reduced so that increased torque was available and the power remained substantially constant. This prevented overloading of the machine and prevented such loading of the system as would bring output shaft to standstill. Control systems of this type are commonly used for hydrodynamic transmission in road-travel automotive vehicles although they have also been used with hydrostatic transmissions by way of suitable controls.
Systems of the latter type, however, do not permit the sensitive control of the speed and positions of the vehicle obtainable with the direct transmission ratio control arrangement described above.
It should also be recognized that it is known, in the control of a hydrostatic transmission, to shift--by an operator-controlled member--the control element of a variable-displacement pump of this transmission from its zero or minimum displacement position into its maximum displacement position and only then to operate the control element of a variable-displacement motor of this transmission to control the transmission ratio thereof. In such systems, the actuating or operator-controlled member is the sole means effecting displacement of the control element of the motor.
Throughout this description, whenever reference is made to the "displacement" of a hydrostatic machine--either a pump or a motor--the fluid-medium throughput per revolution is intended. This corresponds to the "stroke-volume per revolution" or the product per revolution where n is the number of pistons or cylinders of the machine, L is the axial stroke per piston over each revolution and A is the cross-sectional area of each cylinder. Naturally, in the case of an axial piston machine of the variable-displacement type L can be varied between L.sub.min or a minimum value of L (usually zero in the case of a pump) and L.sub.max corresponding to the maximum value.